17.1 Tensile Testing
Initial tensile testing was conducted on February 6, 2020 in 192-328 at 12:00 P.M.. Three testing attempts were conducted, and all three tests failed.
We measured and cut the 100 lb rated fishing line to be 6 inches, and made 5 samples. For the first round of testing, we secured the fishing line between the tensile grips, and ran the first test. However, as seen in Figure 23, the stress concentration was too high and the fishing line snapped just below the tensile grips. We then attempted to secure the fishing line by taking off the tensile grips, and tying the line around the pins that secure the grips into place, as seen in Figure 25. However, this test also failed due to the fact that the knot slipped. We tied it with a fisherman's knot, and the double uni knot (which is the best knot for our specific fishing line), and both tests failed at around 20 lbs due to the knot slipping. Finally, we removed the pins and tied the wire around the holes in the instron, however this test also failed. The internal edges were too sharp and cut the wire during the test, and so we need a new method in order to proceed with tensile testing.
Figure 22 & 23. Tensile testing 02/06/20, attempt 1.
Figure 25. Tensile testing 02/06/20, attempt 2.
17.11 Conclusion:
Figure 27. Tensile Testing, 02/06/20
All tensile testing failed.
The knot in the wire could not withstand a great force, as seen in Figure 27. The loads varied greatly as the knots slipped, and therefore all testing was inaccurate and therefore failed. Because of this, we are going to change materials to Loo’s and Company medical grade wire, as noted in our detailed design II section. This wire was given to Andrew as a free sample at a medical device convention in Pomona. We have also received the testing documents for the wire, and have decided to move forward with it. If we have time next quarter and the resources to purchase more wire, we will perform our own tests, however we will not be able to do so before March 17.
17.2 Compression Testing
Initial compression testing was conducted on February 20, 2020 in 192-328 at 12:30 P.M.
Proximal pieces were tested first. We tested three parts in two different directions. The pieces were secured by using extra fishing line to tie the piece around the bottom compression plate. In the first orientation, the proximal piece failed at 38 N, due to a crack along the holes, seen in Figure 29. During the second test, the proximal piece slipped at initial contact with the top compressive plate because the wire was not tight enough, so the test was stopped at around 4 N, the part was resecured and the test was run again. The piece minorly cracked at 45 N, but the test was run until the maximum load was applied (500 N). The part had a crack seen in Figure 31, but the structural integrity of the part was not completely compromised, as it was still able to withstand 500 N. The third test was run, and there was a minor crack at 50 N (seen in Figure 33), but the test was run until the maximum load was achieved. This test also showed that the part
could still withstand a load of 500 N.
Figure 30 & 31. Proximal compression testing 02/20/20, orientation 1, attempt 2.
The proximal pieces were then tested in another direction; pressure from the upper compressive plate was applied to the outer portion of the proximal piece. They were also secured using extra fishing line, and tied securely to the bottom compressive plate. The first test was run, and the part failed at 70 N along the support material, as seen in Figure 35. The second test was run and the part failed at 170 N. As seen in Figure 37, the inner portion of part shattered. Finally, the third test was run, and the part had a minor crack at 47 N, another crack at 100 N, but still withstood compression until the load maxed out at 500 N.
Figure 36 & 37. Proximal compression testing 02/20/20, orientation 2, attempt 2.
Figure 38 & 39. Proximal compression testing 02/20/20, orientation 2, attempt 3.
The next testing was conducted on the distal pieces. The distal pieces were connected with the internal cam inside. We wanted to test the distal pieces in two directions. The first compressive test was run with the distal piece laying on its side, and the load was set to 2 N in order to hold the part in place and prevent slipping. There were three trials of this test run, and all three tests passed. They withstood the maximum load the instron could give, of 500 N. All three parts can be
seen in Figure 41, and none had any defects. When we attempted to run the test in the second orientation (with the compressive plate putting pressure on the top of the distal piece), the parts slipped. We will need to retest the pieces once the set screws are able to hold the parts together.
Figure 40. Distal compression testing 02/20/20, orientation 1.
Figure 42. Distal compression testing 02/20/20, orientation 2, attempt 1.
Figure 43. Distal compression testing 02/20/20, orientation 2, attempt 1.
The second round of distal compression testing was conducted on February 26, 2020 in 192-328 at 12:00 P.M.
The distal pieces were connected with two set screws and two screws with the internal cam inside. The orientation of the test can be seen below in Figure 44. The first test conducted failed, because the set screws came out of their internal holes. The test failed due to the internal components; the two distal parts were unharmed as seen in Figure 45. The second test made it to 500 N, but the part is deformed as seen in Figure 46 and 47. The set screws and screws stayed in tact. For the third test, the distal piece also withstood 500 N of force, but the part deformed and there were lots of cracks in the material, as seen in Figure 48 and 49. The set screws and screws stayed in tact; the part failed because of the material.
Figure 44 & 45. Distal compression testing 02/26/20, orientation 2, attempt 1.
Figure 48 & 49. Distal compression testing 02/26/20, orientation 2, attempt 3. 17.21 Conclusion:
Proximal Pieces:
Figure 51. Proximal Compression Testing for Orientation 2, 02/20/20.
Proximal testing in orientation 1 and orientation 2 failed.
Each test had cracks either along the holes, or where the part internally loses thickness. The tests still were able to withstand a force of 500 N, so we are feeling confident that if we change materials so something slightly stronger, it would be able to still withstand that force, and possibly not crack. We also tested the pieces without anything supporting the inside of the prosthetic, which is unrealistic for when the patient is wearing it. In reality, the residual of the thumb will be supporting the prosthetic where the material gets thinner and cracked, so we do believe that the prosthetic when he is wearing it will be able to support a greater force without failing.
Distal Pieces:
Figure 52. Distal Compression Testing for Orientation 1, 02/20/20.
Figure 54. Distal Compression Testing for Orientation 2, 02/26/20
Distal testing in orientation 1 was a success and distal testing in orientation 2 failed.
Orientation 1 (force from the side) succeeded. The parts were able to withstand a force of 500 N
without failing. There were no cracks noticeable, and the parts retained their structure as seen in
Figure 41 above. Figure 52 represents the Load Vs. Extension for the distal pieces in orientation
1. It is seen that as the instron compression plates extended and the piece was compressed, there
were no dips indicating any cracks in the material, and all three tests passed the test as they made
it to 500 N. Orientation 2 (force from the top) failed. In the first test, it was the internal components such as
the set screw that caused the part to fail. The internal parts came out and so the distal piece was
not intact and able to withstand 500 N. In the second and third test, the two halves of the distal
piece and internal parts stayed intact, but the PLA material was cracked. Figure 54 shows numerous cracks that the second and third test experiences. There were numerous cracks (which are shown as dips in the graph), which can be seen in Figures 46 and 48. The patient will most likely not experience a force of 500 N directly on the top of the distal piece,
and so we were expecting the test to fail. We are planning on changing materials, and so we are
hoping that next quarter the stronger 3D printed material will be able to withstand a higher force, without cracking.
17.3 Hyper-Extension Testing
The test was conducted by attaching the 3D printed hand model to a block of wood, and securing the prosthetic assembly over top of the hand/wood. The hand was secured horizontally in soft jaws of a vice, as seen in Figure 55 and 56. A bucket (measured at 14.6 g) was secured to the prosthetic using extra of the fishing line, as seen in Figure 58. The bucket was secured along the axis of rotation between the proximal and distal pieces, and 16 oz of water were slowly poured into the bucket.
The part failed at 13 lbs (1 lb of the bucket, and 12 additions of 16 oz of water). The part failed due to a crack in the material. We were expecting the stitching holding the proximal piece to the glove, or the hinge to fail. However, we are reprinting our final product that we will deliver to our patient in a stronger material.
Figure 56 & 57. Compression Testing Orientation.
Figure 60 & 61. Tools for Testing.
Figure 62. Hyperextension Testing, 03/05/20. 17.31 Conclusion:
water, as seen above in Figure 62. Due to the failure of the other tests,we anticipated that the material would also fail during the hyper-extension test. We are changing the material of our final product to carbon fiber, and therefore believe that the stronger 3D printed material will be able to withstand a greater force without cracking.
17.4 Water Testing
We have decided not to conduct the water testing. We are switching to the 3D printers at Cal Poly Pomona that can print with carbon fiber impregnated nylon. Therefore, there is no need to conduct a test to determine if PLA will withstand 30 minutes of a water bath.